The present invention generally relates to structural fasteners and length measurements, more particularly, to a fastener grip length selector, and to a method for precisely measuring an assembly thickness in a clamped-up condition.
Structural fasteners come in varying length to accommodate any thickness of structures. The thickness of an assembly may vary over an area and, consequently, each hole where a fastener is to be installed needs to have its length measured in order to select and install the correct length fastener. Selecting the correct grip length of a fastener is critical in many applications, such as aircraft airframe installation. If a structural fastener is used that does not have the correct length, it may not be possible to install the fastener correctly and the joint strength may be reduced. To ensure the correct installation of fasteners, numerous types of measuring devices are available to measure the length of the hole where the fastener is to be installed. Typically, such measuring devices are gauges that include a measuring scale and that are formed like a fishhook. Such gauges can be hooked onto the backside of a fastener hole, and the number at the scale that is even with the front side of the assembly indicates the length of the hole. Usually, if the reading is between increments, the next higher increment is used for the selection of the fastener to be installed in this hole. The gauging process of the fastener holes is typically done manually.
Prior art gauges may include, for example, a grip length gauge and a digital gauge. The grip length gauge is simply a flat metal blade with a hook on the end. The gauge is inserted through a hole so that the hook contacts the inner mold line surface of an assembly. The metal shaft extends through the top of the hole. On the side of the shaft are markings that correspond to the grip length of the hole. The shortcoming of this gauge is that the outer mold line surface of the assembly must be visually aligned to the markings on the shaft, allowing for errors. The digital gauge may include a spindle that has a contact at the end. The contact may resemble a hook. The spindle is inserted through the hole until the clamping shaft of the gauge rests on the outer mold line of the assembly. The gauge is positioned such that the contact rests on the inner mold line surface of the assembly. The grip length is then the distance from the hook to the clamping shaft. The shortcoming is that the assembly is not in a clamped-up condition at this hole, and that the spindle mechanism can become contaminated with excess liquid shim that has been squeezed into the hole, which is not desirable.
In an installation process, for example, the installation of an aircraft airframe, the step of measuring the length of fastener holes is a very important step to ensure the correct selection of the permanent fastener grip length and the correct installation of permanent fasteners, but it is also a time consuming step that slows the product flow. Furthermore, should a fastener be installed incorrectly because of use of an incorrect fastener length, the fastener would need to be removed and replaced, resulting in significant rework and potential damage to the assembly.
Many assemblies, for example, in the aerospace industry or in the automobile industry, require temporary fasteners to be installed. Temporary fasteners hold the material in place until it is ready to be fastened permanently. Temporary fasteners are typically installed in assemblies that consist of a plurality of panels, such as a skin, a sealer or shim, and a substructure, for example, an aircraft airframe assembly. In an installation process, for example, of an aircraft airframe assembly, temporary fasteners may be inserted into fastener holes in a first step. The temporary fasteners might need to be removed one by one and the length of each fastener hole might need to be measured in a subsequent step before the permanent fastener can be selected and installed. This process is time consuming and increases the cycle time.
Existing temporary structural fasteners include, for example, Wedgelock™ temporary fasteners manufactured by Monogram Aerospace Fasteners, Los Angeles, Calif., U.S.A. Wedgelock™ temporary fasteners are frequently used, for example, in the aircraft airframe installation process to temporarily hold an assembly, such as substructure, sealant, and outer mold line skin, together. The Wedgelock™ temporary fastener is similar to the temporary fastener 10 illustrated in FIG. 1. The temporary fastener 10, as illustrated in FIG. 1, is a temporary blind fastener that has a cylindrical body 11. The cylindrical design eliminates fastener-to-tool orientation problems. Still, the temporary blind fastener 10 does not necessarily need to be cylindrical—e.g. could be hexagonal, octagonal, etc. The temporary fastener 10 further includes a captive nut 12, double lead thread (not shown), and larger geometry bearing wires 13. Utilization of a captive nut 12 provides a positive disengagement, allowing unobstructed removal of the fastener even if used in wet sealant applications. The double lead thread design (not shown) further enhances overall installation and removal efficiency while the large bearing area makes this type of fastener the optimum choice for both composite and metallic structures. The temporary fastener 10 is a power operated reusable temporarily fastener and may be installed and removed manually, automatically, or by a robot. Using the prior art temporary fastener 10 may improve the installation and removal efficiency for temporarily fastening structures, but the temporary fastener 10 still needs to be removed from a structure to allow access to the fastener hole to be measured using a separate measuring device.
As can be seen, there is a need for a measuring device that enables accurate and efficient determination of a fastener hole length. Furthermore, there is a need to enable the measurement of a fastener hole while a temporary fastener is installed in this hole. Moreover there is a need to enable correct selection of the fastener grip length of the fastener to be installed.
There has, therefore, arisen a need to provide a length-measuring device that enables accurate determination of a fastener hole length without removal of the temporary fastener from the fastener hole. There has still further arisen a need to provide a method for precisely measuring the thickness of a temporarily fastened assembly and for selecting the correct fastener grip length for a fastener to be installed permanently.